driftwood lowers your water pH level. I dont think it'll rot in your tank. I think its a special type of tree. I found it makes the water get a yellow tinge, but i've heard that can be removed by boiling it.
If you buy aquarium safe driftwood and soak, boil, and rinse it A LOT of times, then it will neither stain your water nor change your pH. However, if you put that driftwood in your water as it is, then it most certainly will stain your water brownish color and will lower your pH a lot. Aquarium safe driftwood should never rot in your tank. Here is another post about driftwood that should be helpful to you:
I have 2 pieces of aquarium safe driftwood it my 30 gallon tank. I soaked, rinsed, and boiled it many many times and for long periods. I put the driftwood in my tank once it completely stopped leaching any color. That's when I knew it was going to be safe to use. It did not change or affect my pH in any way.